Process for production of as-cast nodular iron



United States Patent 3,492,118 PROCESS FOR PRODUCTION OF AS-CAST NODULAR IRON Robert L. Mickelson, Cambridge, Ohio, assignor, by mesne assignments, to Foote Mineral Company, Exton, Pa., a corporation of Pennsylvania No Drawing. Filed May 24, 1966, Ser. No. 552,438 Int. Cl. C21c 1/10 U.S. Cl. 75130 4 Claims ABSTRACT OF THE DISCLOSURE Process for the production of as-cast nodular iron comprising forming a molten bath of cast iron; adding a rare earth-silicon-iron alloy containing up to about 50 percent rare earth, at least one-half of which is cerium, and at least 25 percent silicon to said bath in an amount sufiicient to introduce about 0.03 percent to 0.15 percent cerium into said bath; pouring castings from said bath, and allowing said castings to solidify.

This invention relates to the production of nodular cast iron and, more particularly, to an improved process for the production of as-cast nodular iron.

Some graphite is always present in cast iron. In ordinary gray cast iron the graphite is dispersed in a flake-like array in a matrix of ferrite and pearlite. In nodular cast iron, or nodular iron, an appreciable percentage of the graphite is in spheroidal form. The mechanical properties of nodular iron, its strength, ductility, and resistance to impact, far surpass the mechanical properties of ordinary gray iron.

It is well-known that the addition of certain elements to a molten cast iron bath will cause the graphite to spheroidize during solidification of the melt. Elements that have been reported to cause spheroidization of graphite in cast iron include lithium, sodium, magnesium, calcium, strontium, barium, cerium, yttrium, beryllium, and lanthanum. However, current commercial production of nodular iron depends almost exclusively upon magnesium as a spheroidizing agent. Cerium and calcium additions frequently are added along with the magnesium to the molten cast iron bath, but these additions serve secondary roles and, under conditions of current use, do not function as spheroidizing agents. Cerium is used to counteract deleterious residual elements in the iron which inhibit spheroid formation. Calcium improves the efliciency of magnesium by decreasing the vapor pressure of magnesium and by combining with some of the sulfur in the Hon.

Although magnesium has generally been considered to be the most suitable spheroidizing agent, there are numerous disadvantages associated with the use of magnesium. The boiling point of magnesium (2025 F.) is considerably lower than the temperature of the molten iron bath at the time magnesium must be added to cause spheroidization of the graphite. Consequently, a violent reaction accompanies introduction of magnesium to molten iron. Many methods of subduing this reaction have been devised. These include alloying the magnesium with nickel or silicon prior to introducing the element or alloy into a ladle of molten iron, introducing the magnesium 3,492,118 Patented Jan. 27, 1970 into molten iron in a pressure vessel, and entraining small magnesium particles or magnesium vapor in a stream of inert gas which is injected into the molten iron. These methods are costly and although they reduce the reaction violence to a degree that makes that addition of magnesium to iron feasible, it is still a hazardous operation.

Several advantages can be obtained by using cerium to effect graphite spheroidization rather than magnesium. The mechanical properties of cast iron improve nearly linearly with increasing cerium contents up to a cerium level between .02 percent and .06 percent. Also residual elements which inhibit spheroid formation can be tolerated at higher levels in cerium-treated nodular iron than in magnesium-treated nodular iron. Most important is the fact that cerium and the other rare earth elements generally found with cerium boil at temperatures greatly in excess of the temperature of the molten iron bath. Thus, negligible volatilization losses are encountered, the process is less sensitive to iron temperature, and the hazards involved in introducing cerium to the bath are far less than when magnesium is added as a spheroidizing agent.

However, the use of cerium has been limited to research for several important reasons. Cerium is believed to be effective in causing only hypereutectic graphite to spheroidize. Magnesium, on the other hand, imparts nodular structure to both hypoeutectic and hypereutectic graphite. In addition, the prior attempts to utilize cerium as a spheroidizing agent have been by means of adding mischmetal to an iron bath. Mischmetal, which is an alloy composed of about 48 percent cerium, 25 percent lanthanum, 15 percent neodymium, 9 percent other rare earths, and up to about 5 percent iron, is ductile and must be sawed, cut, or cast into pieces of convenient size for adding to the iron. Therefore, it is difiicult to size mischmetal into small particles which permit the addition of exact amounts of cerium to the bath.

Finally, cerium-treated cast iron is extremely sensitive to both the rate of cooling and the number of spheroids present per unit volume of iron. It is generally believed that the graphite spheroids formed in hypereutectic cast iron treated with cerium according to present practices are enveloped in an austenite coating above the eutectic temperature. As the temperature of the melt is lowered and the eutectic transformation proceeds, carbon migrates through the coating of austenite and deposits on the spheroid. The extent to which this migration and deposition occurs depends upon the cooling rate and the number of spheroids present per unit volume of iron. If the iron contains a sufficient number of spheroids and if the cooling rate is slOW enough to permit carbon diffusion to proceed, all of the carbon liberated during the eutectic transformation will deposit on the initial hypereutectic graphite spheroids. However, if the diffusion path is too long because of relatively too few hypereutectic graphite spheroids or if the rate of cooling through the eutectic region is too rapid, some of the eutectic graphite will fail to reach its destination and will precipitate as short flakes known as vermicular graphite. Very rapid cooling through the eutectic region also results in the formation of iron carbide which, if it decomposes, liberates vermicular graphite. Vermicular graphite forms frequently by these mechanisms in cerium-treated nodular iron. The mechanical properties of nodular iron are adversely affected by the presence of vermicular graphite.

I have developed a method of adding cerium to a cast iron bath as a graphite spheroidizer which overcomes the difficulties previoulsy encountered in the use of cerium as a graphite spheroidizer. My invention provides a method of manufacturing nodular iron which yields iron The rare earth-silicon-iron alloy may be added to the molten bath of cast iron in a gas-fired or electric melting furance just prior to tapping. However, I prefer to add the alloy to a ladle being filled with the molten iron. Carbon or graphite ladle linings insure the highest reof a uniform structure, consistent properties, with an 5 covery of cerium but either acid or basic linings can be outstanding efficient use of the nodulizing agent. My used. The alloy particles should be of a size that will process is not dependent upon the iron temperature and pass through a b. inch or smaller screen. Subsequent to is not adverse affected by the presence of deleterious rethe addition of the rare earth-silicon-iron alloy, the bah sidual elements in amounts which previously had been conmay be inoculated with a silicon base alloy such as 75 sidered prohibitive. Finally, it provides a means of utilpercent ferrosilicon, although this addition is not necesizing cerium to produce nodular iron essentially free sary. from vermicular graphite. To demonstrate the effectiveness of my process, I have Briefly stated, my invention comprises the steps of prepared heats of cast iron in a l00-pound induction furforming a molten bath of cast iron composition, adding nace. The furnace charge consisted of low sulfur pig to the molten iron bath an alloy of rare earth, silicon iron, Armco iron, and ferrosilicon. The iron was tapped and iron in an amount suflicient to introduce about 0.03 from the furnace into a 100-pound ladle and during the percent to 0.15 percent cerium in the bath, pouring tapping operation, a rare earth-silicon-iron alloy sized castings from the bath and allowing the castings to solidiminus inch or smaller was added to the ladle. The fy. The rare earth-silicon-iron alloy should contain up to rare earth-silicon-iron alloys utilized were prepared by 50 percent rare earths, at least half of which is cerium, reducing silica and concentrated rare earth ore with carand at least percent silicon. bon in a submerged arc smelting furnace. Iron was in- Although the exact mechanism by which my novel troduced by adding steel scrap to the furnace charge. method provides a superior nodular cast iron is not Following the tap, one pound of 75 percent ferrosilicon known, it is believed that the high degree of nodularity 25 was stirred into the iron bath. The iron was then poured is due to the growth of very large number of graphite into dry sand molds to obtain standard ASTM 1 inch spheroids nucleated by silicon-bearing cerium additions. keel block castings and other castings of varying dimen- It is believed that the dissolution of silicon from the rare sions. Standard ASTM 0.505 inch diameter tensile speciearth-silicon-iron alloy causes the molten metal immedimens were machined from the keel blocks. ately adjacent to the alloy particle to become extremely In a first test series an iron bath containing 3.7 perhyper'eutectic in composition. Since cerium appears to cent to 4.1 percent carbon, 2.4 percent to 2.8 percent spheroidize hypereutectic graphite only, it is believed that silicon, less than 0.1 percent manganese and less than the hypereutectic zones around particles of dissolving 0.03 percent phosphorous was treated as described above alloy enhance the spheroidizing effect of cerium. The with a rare earth-silicon-iron alloy containing 10 percent actual mechanism by which graphite nuclei originate cerium, 39 percent silicon, 7 percent other rare earths, and becoming encased in austenite is not understood and the balance iron. Data pertaining to the first series thoroughly. However, spheroid nuclei commonly are beof tests and to the properties of the resulting cast iron lieved to be either minute particles of graphite expelled are set forth in Table I.

TABLE I Keel block properties Final iron composition, percent Treat- Ultimate me'nt Percent Percent Thinnest tensile Yield Test Carbon temp., Ce Ce Percent carbide-free strength, strength, Elongation, No. equiv. S Ti Pb F. added retained nodularity section, in. 1,000 p.s.i. 1,000 p.s.i. percent 4.96 0.03 0. 02 0. 001 2,700 0.15 0.05 95 0.1 N.D N.D. N.D.

1 Percent carbon equivalent=percent C+1/% Si.

from the melt or crystals of a carbide of the spheroidizing As shown in Table I consistently high tensile strength element. Either of these origins can be facilitated by a 55 was obtained on the as-cast nodular iron made accordsilicon-rich hypereutectic zone of molten iron that would be supersaturated with carbon.

In practicing the present invention, cerium is the only essential spheroidizing element. However, most of the rare earth-silicon-iron alloys that were used contained about 0.6 part of lanthanum, 0.2 part of neodymium and 0.1 part of other rare earth for each unit of cerium. Rare earth elements other than cerium apparently serve to a small extent in deoxidizing and desulfurizing the iron and thereby may improve the cerium efiiciency.

The efficiency and effect of cerium generally improves as the weight percentage ratio of silicon to cerium is increased. Within the scope of my invention the silicon content of the cerium alloy should be greater than about 25 percent and preferably greater than 30 percent. The practical minimum cerium content of the alloy is approximately 2 percent. However, I prefer to add alloys that contain 10 percent to 25 percent cerium because alloys having levels of cerium within this range perform we a o fer maximum economy, l

ing to my invention. Variation in the iron carbon equivalent within the range of 4.55 percent to 4.89 percent and variation of the treatment temperature from 2500 F. to 2800 F. caused little variation in the iron properties. In Test No. 7, an 0.03 percent addition of sulfur was made to the cast iron and a high degree of nodularity was obtained even though the iron contained 0.03 percent retained sulfur. It was, however, necessary to add 0.15 percent cerium to this bath to obtain this high degree of nodularity. In Test No. 8, there was an addition of 0.11 percent titanium and 0.09 percent lead. Addition of these amounts of titanium and lead to the molten iron which is to be treated with magnesium inhibits graphite spheroidization. Nevertheless the results of Test No. 8 show that the titanium and lead additions to the bath had no effect upon strength and caused only a minor decrease in ductility.

A second series of tests was conducted on iron baths having a composition of 4.1 percent to 4.3 percent carbon, 3.1 percent to 3.2 percent silicon, 0,60 percent manganese,

0.01 percent sulfur, 0.04 percent phosphorous, 0.15 percent copper, and 0.16 percent chromium. The melting and treating procedures for this series of tests were identical to the procedures that were used for the first series and the same rare earth-silicon-iron alloy was used. How- 6 The results of Test Series No. 3 show that the addition of a rare earth-silicon-iron alloy to a cast iron bath as taught by my invention, gives substantially improved properties to the as-cast iron over the untreated cast iron. While I have described my invention in terms of the ever, the amount of cerium added was varied from 0.03 5 presently preferred method, it is to be understood that it percent to 0.06 percent. This constituted an addition of may be otherwise practiced within the scope of the folsix to twelve pounds of the rare earth-silicon-iron alloy. lowing claims. Data pertaining to the second series of tests and to the I claim: properties of the resulting cast iron are set forth in 1. Aprocess for the production of as-cast nodular iron Table II. comprising the steps of:

(A) forming a molten cast iron bath,

TABLE II Keel block properties Final iron Ultimate carbon Thinnest tensile Yield Test equiv., Treatment Percent Ce Percent Ce Percent carbide-free strength, strength, Elongation, Brinell No. percent temp., F. added retained nodularity section, in. 1,000 p.s.i. 1,000 p.s.i. percent hardness Without treatment according to this invention, iron having the composition shown in both Test Series No. 1 and No. 2, exhibits less than 20,000 p.s.i. ultimate tensile strength and no ductility. The as-cast nodular iron made according to this invention exhibits outstanding tensile strength and ductility and the properties were very consistent. As shown, 0.1 inch thick castings that were free of massive carbides were poured from each heat.

A third test series was conducted in the same manner as Test Series No. 1 and No. 2 using a rare earth-siliconiron alloy having a composition of 22 percent cerium, 45 percent silicon, 19 percent other rare earths, one-half percent calcium, one-half percent aluminum, and the balance iron and incidental impurities. The composition of the iron in this test series was the same as the composition of the iron in the second test series. The castings were performed using the same procedures utilized in Test Series No. 1 and No. 2 as described above. Additional data pertaining to Test Series No. 3 and to the (B) adding to said bath a rare earth-silicon-iron alloy containing up to about percent rare earth, at least one-half of which is cerium, and at least 25 percent silicon in an amount sufficient to introduce about 0.03 percent to 0.15 percent cerium into said bath,

(C) pouring castings from said bath, and

(D) allowing said castings to solidify.

2. The process set forth in claim 1 wherein said rare earth-silicon-iron alloy contains from 10 to 25 percent cerium and from 35 to 50 percent silicon.

3. The process set forth in claim 1 wherein said rare earth-silicon-iron alloy has a composition of 10 percent cerium; 39 percent silicon; 7 percent other rare earhts; and the balance iron and incidental impurities.

4. The process set forth in claim 1 wherein said rare earth-silicon-iron alloy has a composition of 10 to 25 percent cerium; 35 to 50 percent silicon; 5 to 20 percent other rare earths; 0 to 1 percent calcium; 0 to 1 percent aluminum; and the balance iron and incidental impurities.

properties of the resulting cast iron are set forth in 45 Table III.

TABLE III Keel block properties Final iron Ultimate carbon Thinnest tensile Yield Test equiv., Treatment Percent Ce Percent Ce Percent carbide-free strength, strength, Elongation, Brinell N 0. percent temp., a ed retained nodularity section, in. 1,000 p.s.i. 1,000 p.s.i. percent hardness References Cited UNITED STATES PATENTS 2,488,512 11/1949 Morrogh 75-123 2,542,655 2/1951 Gagnebin 75123 2,970,902 2/ 1961 Alexander 75123 OTHER REFERENCES Morrogh, H.: Nodular Graphite Structures Produced in Gray Cast Irons, in American Foundryman, pp. 91-

106, April 1948.

L. DEWAYNE RUTLEDGE, Primary Examiner I. E. LEGRU, Assistant Examiner U.S. Cl. X.R. 

